mmmm.. I've seen explanations of both gerundive and gerundive that talk about ablative expressing cause. Now I know one simple rule might be that since the Gerundive is an adjective that 'puero tuendo' must be a gerundive as there is a noun being described...' but there seems to be an overlap between gerundive and gerund in this regard. Can anyone point at the above sentence and give me a decisive reason why it's one or the other...?

My initial view was that it's a gerundive but only because I've seen examples about Caesar being killed which seem to follow a similar construction.

I think that tuendo is a gerund in the ablative case because it is describing how, or by what means, Dido is burned up. Now, gerunds, because they are verbal nouns, can take an object. Frequently in Latin, the object of a gerund is put into the same case as the gerund itself. This is what looks like is happening with puero - it is ablative because it is the object of an ablative gerund.

I think that tuendo is a gerund in the ablative case because it is describing how, or by what means, Dido is burned up. Now, gerunds, because they are verbal nouns, can take an object. Frequently in Latin, the object of a gerund is put into the same case as the gerund itself. This is what looks like is happening with puero - it is ablative because it is the object of an ablative gerund.

Vale!

If it were a gerund it would be puerum tuendo, though. puero tuendo is the gerundive equivalent of that.

The gerund is an active verbal noun. It's like an infinitive, except it has genitive, dative, and ablative forms, whereas an infinitive can only be nominative or accusative.

The gerundive is a passive verbal adjective. You can think of it as another participle, a future passive participle.

The gerund can take an object, but something about that construction offended the Roman ear, so they replaced the gerund with a gerundive. Logically, your initial example would be "puerum tuendo" (i.e., "from watching the boy"), but the logical object ("puer") is instead put in the gerund's case, and the gerund is replaced with a gerundive. This is called "gerundival attraction".

Your second example does not make sense as it stands. With a gerund, it would be: "Caesarem interficiendo, Brutus et Cassius rem publicam restituerunt", "Brutus and Cassius restored the republic by killing Caesar." This seems perfectly sensible, but rather than a gerund with an object we would actually be more likely to find a gerundive: "Caesare interficiendo, ..."

It will be more obvious that these are gerundives if we look at examples with plural or feminine nouns, since the gerund, being neuter and singular, cannot take the range of endings that the gerundive can:

This construction always struck me as one of the weirdest things in Latin grammar, and I find I can only really understand or translate it by mentally replacing the gerundive either with the gerund or with the perfect participle.